Hydraulic fluids used in the hydraulic systems of aircraft, for example, have the tendency to collect and become contaminated with various pollutants and damaging impurities, especially including water and corrosive substances dissolved therein. Such pollutants or contaminants can lead to damage, and particularly corrosion damage, within the respective corresponding hydraulic system in which the hydraulic fluid is used. Therefore, it is necessary that the hydraulic fluid used in such hydraulic systems must be replaced or cleaned to remove these contaminants at certain maintenance time intervals. For convenience, such hydraulic fluid is sometimes referred to simply as hydraulic oil herein.
In order to carry out such a cleaning of the hydraulic oil, stationary apparatus or equipment is typically used. As a consequence, it is necessary to remove from the respective hydraulic system the dirty or contaminated oil that is to be cleaned, carry the dirty oil to the cleaning apparatus located away from the respective hydraulic system, and then after the cleaning operation it is necessary to return the cleaned oil into the hydraulic system.
An apparatus of this known type is described in U.S. Pat. No. 4,604,109, the disclosure of which is incorporated herein by reference. An essential element in the known apparatus is a centrifugal disk arrangement that centrifugally spins the oil so that the water dissolved within the oil can again be extracted therefrom, while damaging particles are simultaneously removed from the oil by means of suitable filters. In the known apparatus, the dirty hydraulic fluid or oil that is to be cleaned is supplied to the centrifugal disk through a central supply arrangement. A vacuum pump is connected to the upper portion of the housing of the centrifugal disk arrangement, so as to evaporate, extract and suck-away water vapor out of the oil, while the bottom portion of the centrifugal disk housing forms a collecting basin for collecting the cleaned hydraulic fluid. Furthermore, the known apparatus includes one container for the contaminated or dirty hydraulic fluid that is to be cleaned, and one container for the cleaned hydraulic fluid.
In using the above described known apparatus for cleaning the hydraulic oil of an aircraft hydraulic system, it is necessary to constantly maintain or store a corresponding quantity of intact oil for carrying out an exchange of the oil, in order to avoid an undesirably long ground time of the aircraft while carrying out the oil cleaning operation. However, in carrying out such a process, a portion of the contaminated or dirty oil will always remain in the hydraulic cylinders of the corresponding hydraulic system, and can thus not be processed for cleaning. As a result, a certain quantity of contaminated oil, and the corresponding contaminants, remain in the hydraulic system of the aircraft even after the oil cleaning procedure has been completed. Moreover, as a further disadvantage, the known apparatus and the method of operating it are inconvenient because the necessary cleaning step cannot be carried out directly at or adjacent to the aircraft.